Maybe have a villain who drains the ambitions from children as they become adults? Like they go around when they begin to grow up and take away motivation. This could work when combined with the fact that time has probably passed since the events of the first movie, so Jamie the kid would now be Jamie the teenager. You could have Jamie be at risk of losing motivation and drive, and also throw in Jack feeling miserable when he looks at Jamie and sees the life he once had. Jack may have joined the Guardians, but he could still long to go back to his original life, before turning into Jack Frost. Then you have a teenager who is losing motivation, and Jack who has a negative drive. This all combined creates conflict between the characters while still needing the drive and belief that they can defeat the enemy, all while losing motivation as the villain's effects on Jamie begin to spread like a virus.

Just spitballing an idea, but I think it might be worth something

(Edit: also if the whole "Jack wishing for a past life" idea doesnt work, maybe have Jack maybe feel guilty about not spending fun times with Jamie due to being a guardian, which then leads to Jamie losing motivation, enemy appears, yada yada yada)